The Question

SuperUser reader Mega wants to know if there is a way to tell if two DVDs are exactly the same or not:

I have two DVDs and if I open the them, then copy the contents to my hard-drive to compare the respective files, it shows no difference. As far as I know, DVDs also have some additional content (such as information saying if the DVD is bootable along with some formatting information I guess).

How can I also check this additional content? Is it possible without additional programs on Windows or Ubuntu?

Are there any methods a person could use to verify whether two DVDs are identical or not?

The Answer

SuperUser contributors mtak and Dithermaster have the answer for us. First up, mtak:

The easiest way to determine if two discs are the same is to run a hash of both of them:

sudo md5sum /dev/cdrom

If the hashes match, then the disks are exactly the same. However, if the hashes do not match, then this will not tell you what is different about them. If even a single bit is different, you will get a totally different hash.

You can check the partition table of a disc with fdisk:

sudo fdisk -l /dev/cdrom

Followed by the answer from Dithermaster:

Just comparing folders and files misses other things about the disc. If you create an .ISO image file from each disc and byte compare them, you would have a better idea if the discs are really the same or not.

Should be noted that MD5 is risky to be trusted for pretty much anything, but not as much as the CRC used by TeraCopy, for instance; for example, using MD5 on passwords is seriously frowned on practise. Myself, I'm always using, in this order:- ssdeep- SHA512- SHA256- SHA128 [very final option]but never MD5 if I can avoid it.

Akemi Iwaya is a devoted Mozilla Firefox user who enjoys working with multiple browsers and occasionally dabbling with Linux. She also loves reading fantasy and sci-fi stories as well as playing "old school" role-playing games. You can visit her on Twitter and Google+.

Published 06/12/14

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